1. Field of the Invention
The present invention relates to a lead frame to be used for assembling a semiconductor device, a method of manufacturing the lead frame, and a method of manufacturing the semiconductor device.
2. Related Art
Lead frames for semiconductor assembly so far developed include the one disclosed in JP-A No. H07-169901. Paragraph [0009] of this document states that depositing a palladium or soft metal strike layer, a Pd—Ni layer, a Pd layer and an Au layer in an ascending order on a Ni layer enables attaining a composite surface structure that can effectively cover a base metal of the lead frame.
Also, paragraph [0015] of JP-A No. H09-017932 states that the composite surface structure on the lead frame is to include a Pd (soft metal) strike layer, a Pd—Ni alloy layer and a Pd layer in an ascending order on the Ni layer, to thereby finish the lead frame with a reduced amount of precious metal or without using Au, while securing sufficient soldering performance of the lead frame surface.
With the conventional composite plated structure on the lead frame of a pre-plated frame type (hereinafter, PPF), however, the lower layer metals other than that of an outermost surface layer, such as Ni, perform grain boundary diffusion when heat load is applied through an assembly process of a semiconductor, and come up into the outermost surface layer. The Ni, which has thus intruded in the outermost surface layer, constitutes a predominant factor that degrades the bonding performance between an Au wire and the lead frame, as well as the adhesion between the lead frame and an encapsulating resin.
The lead frame for semiconductor provided with the composite surface structure according to Japanese Laid-open patent publication No. A-H07-169901 and Japanese Laid-open patent publication No. A-No. H09-017932 has its entire surface covered with the Pd plated layer and so forth, and therefore it is impossible to decide whether Ni is present in the outermost surface layer by appearance inspection, after undergoing the heat load through the assembly process.
In the actual assembly process, consequently, defects are revealed such that the Au wire cannot be bonded to the lead frame with the composite plating, and that the lead frame with the composite plating is not completely adhered to the encapsulating resin and comes off.